The present invention relates to a control circuit for moving a pen up/down in a drawing apparatus such as an X-Y plotter or a drafting machine under the control of a current supplied to a drive coil.
In a conventional drawing apparatus such as an X-Y plotter, a pen is brought into contact with (pen-down) a surface of a recording sheet by moving a pen holder downward upon operation of a plunger-solenoid, and the pen is lifted (pen-up) by a biasing force of a return spring to the initial pen-up position. According to this system, a control operation is simple, however, an impact of the pen tip brought into contact with the recording surface is increased, and impact sound is also increased. In addition, it is difficult to obtain an optimal pen pressure in the pen-down mode. Furthermore, a residual magnetic field is often left in rotary and stationary iron cores of the plunger-solenoid, and the response speed of the pen is decreased over time. Another conventional X-Y plotter employs an impact damping means such as an air damper. However, the air damper decreases the response speed.
In order to solve those problems, a conventional pen control circuit shown in FIGS. 1 to 3 for electrically controlling a current supplied to a pen drive coil has been proposed (Japanese Patent Prepublication Nos. 59-2092207 and 58-182469) by the same applicant as that of the present invention. Referring to FIG. 1, a pen carriage housing 1 includes a sliding unit 1A reversibly slidable along a guide shaft 2 in the Y direction, and a pen holder unit 1B, reversibly movable in the Z direction (pen-up and pen-down directions) along a sliding shaft 3, for holding a pen 4. A drive coil 5, a speed detection coil 6, and a position detector (e.g., a Hall element) 7 are arranged in the pen holder unit 1B. A pair of permanent magnet plates 8A and 8B are mounted in the pen carriage housing 1 to oppose each other, as shown in a sectional view (FIG. 2) taken along the line A--A of FIG. 1. The drive coil 5, the speed detection coil 6 and the position detector 7 are arranged in a gap defined by the pair of permanent magnet plates 8A and 8B. The pen holder 1B is vertically moved along the pen-up/-down direction in accordance with a polarity of a current supplied to the drive coil 5, thereby controlling a driving force of the pen holder 1B. A voltage proportional to the speed of the pen-up or -down operation is detected by the speed detection coil 6. A change in magnetic field from the permanent magnets 8A and 8B along the Z direction is detected as a position signal by the position detector 7. Reference numeral 9 denotes a return spring for a pen-up operation; 1C, a stopper for determining a pen-up position; and 10, a recording sheet.
In the pen control circuit, as shown in FIG. 3, a sum signal from an adder 13 for adding detection signals from a current detector 12 and the speed detection coil 6 and an output from a phase compensator 14 are fed back to a current control circuit 11. A selector 16 selects either the position detection signal from the position detector 7 or a set value from a pen pressure setter 15 as a drive current setting value. When a control signal from a pen-up/-down signal generator 17 represents the pen-down mode, feed-back control is performed in response to the drive current setting value from the selector 16. However, when the control signal represents the pen-up mode, feed-back control is performed in response to a predetermined drive current setting value.
In the pen-down mode, the selector 16 selects a pen position signal VA from the position detector 7 until the pen comes close to the recording sheet from a pen-up position PA and reaches position PB, as shown in FIG. 4. The pen position signal VA serves as the drive current setting value. However, when the pen comes close to the recording sheet, i.e., passing the position PB, the selector 16 selects a pen pressure setting signal VB from the pen pressure setter 15. In this case, the pen pressure setting signal VB serves as the drive current setting value. The phase compensator 14 serves as a delay element. Upon operation of the phase compensator 14, the signal VA has a large magnitude during the initial drive period and has a relatively large drive current with a zero feedback component (phase delay), thereby improving the pen-down speed. When the pen tip comes near the position PB, the magnitude of the signal VA is decreased. At the same time, a damping effect of the speed detection signal is increased to decrease the impact of the pen tip at the time the pen is brought into contact with the surface of the recording sheet.
In the pen-up mode, the predetermined drive current setting value with a polarity opposite to that in the pen-down mode is supplied to the current control circuit 11 in accordance with the pen-up instruction from the pen-up/-down control signal generator 17. The pen is moved upward in response to the pen-up current cooperating with a biasing force from the return spring 9.
In the conventional pen control circuit described above, the pen pressure setting value VB is changed over a wide range to obtain an optimal pen pressure corresponding to a specific type of pen 4 (e.g., a fountain pen, a felt-tip pen, or a water ball-point pen). Assume that the setting value VB is set to be a low value VBL or a high value VBH, as indicated by the alternate long and short dashed line in FIG. 4. When constants and gains of the respective components of the control circuit are set to obtain a suitable response speed and a small impact for the setting value VB, the drive current setting value corresponding to the high value VBH is continuously supplied at a position where the pen tip does not come sufficiently close to the recording sheet. The braking effect following speed detection is then decreased to increase the impact. However, when the drive current setting value is the low value VBL corresponding to the position signal VA and the speed detection braking effect cause slow pen movement to a point where the pen comes excessively close to the recording sheet, thereby decreasing the response speed.
In addition to the problem presented by pen pressure presetting in accordance with the different types of pens, a conventional control circuit performs the pen-up operation by cooperation of a predetermined drive current and a biasing force of a return spring. Therefore, variations in return spring characteristics cause variations in the pen-up mode. Furthermore, even if the braking effect following speed detection is delayed and the gain of the position detection signal is increased so as to decrease the impact and increase the response speed in the pen-down mode, braking control in the pen-up mode cannot be performed. Furthermore, since a large current is often supplied to the pen-up position according to position control characteristics of the arrangement, the pen tip may have a high impact, a loud impact sound may be generated, and ink may drip from the pen tip.